CO2 Refrigeration: A Growing Trend in the Supermarket Industry

Carbon dioxide (CO₂) is a natural component of the Earth’s environment, that's needed by plants in order to carry out photosynthesis, making it a necessary component of the atmosphere. In addition to its purpose in nature, CO₂ has many applications used in commercial industries such as food and chemical. For instance, CO₂ has been used in the refrigeration business for well over 100 years. But its use in commercial applications was significantly reduced after the introduction of synthetic refrigerants. Today, however, the popularity of CO₂ is reemerging among supermarket refrigeration equipment users throughout North America.

So why the new sudden interest? Well, there are several reasons for this and we’ll discuss them a little later in this article. But for now, let’s start by first explaining how CO₂ can be used in a refrigeration system.

There are two ways CO₂ can be used in supermarket refrigeration.
One is that of a secondary application (also known as "flooded coil" or "liquid overfeed"). Here, CO₂ is circulated through and partially evaporated in the refrigerated fixtures by means of a liquid CO₂ pump. This method uses the latent heat of CO₂ for cooling thereby requiring very little, or insignificant, pumping energy. This is applicable to both low temperature loads (those loads below 0F) and medium temperature loads (those loads above 0F).

The second method for utilizing CO₂ for refrigeration is by direct expansion. In this method, CO₂ is circulated by means of a vapor compression cycle either for low temperature loads only (in a sub-critical cascade cycle) or for both low and medium temperature loads in a transcritical cycle. The latter is more prevalent in Europe and is not currently in use in the U.S. Chances are you're more likely to see CO₂ in a cascade system in either a secondary low and/or medium temperature system or low temperature direct expansion system. With this reemerging technology, it’s important that refrigeration equipment technicians prepare for installing and servicing these type systems sooner than later. It’s no secret that today’s contractor needs to be more technically astute with the increasing use of electronics, floating control logic, variable capacity control and the ever emerging refrigerant choices. Well this is no different when it comes to CO₂.

The operating pressures are higher than what you would typically see in a supermarket. This is nothing to be afraid of.

So where should you start? First, it’s very important that an individual looking to improve their knowledge of systems and CO₂ should at least have a sound understanding of how a standard direct expansion system operates. Someone who is a beginner or novice should focus on improving their abilities with standard direct expansion systems and then move on to CO₂. With that said, the Internet is the best place to start. You can find hundreds of documents, articles, books and other resources that offer theoretical, practical, safety, and even start up guidelines for systems using CO₂. There are a lot of resources that may have an ‘industrial’ slant to them or be based on experiences in Europe, but the principles and applications are virtually the same to that of a commercial installation. I would also recommend starting with professional organizations that have subject matter expertise in CO₂.

Here's a short list of organizations you can start with:

Next, I would recommend studying various component manufacturers’ literature on their CO₂ product offerings to gain a better understanding of the differences of how their CO₂ components differ from their standard HFC components. This is especially true of the CO₂ compressors. There’s no substitute for practical experience. Once you've built a solid foundation of knowledge about CO₂, you need to get some hands-on experience with it in an actual operating system.

Most end users have a list of approved contractors that they work with. If you have a customer that you do regular business with, I encourage you to work with that end user to do a test store. This will be a great opportunity to gain practical experience, improve your company’s capabilities and keep up with the latest trends in the industry. I also recommend that you view your first project as a learning opportunity. Sure it’s a little different, but from what you studied, you'll find that it operates much the same way as a standard DX system or a glycol system does (depending on how the CO₂ is used to refrigerate). And of course, you will have the support of the equipment manufacturer to help during the start-up process since it is still relatively new to the industry. I would also recommend networking with other contractors to gain access to an installation. With the growing number of CO₂ installations across the U.S., chances are that a system will be, is, or is going to be installed in a location that you could get to relatively easily.

Differences When Compared to Standard Systems
With a CO₂ system in operation, you can expect a few things to be different from a standard system. There are several major differences that you can expect to see:

• First, your line sizes are going to be much smaller and typically of a thicker material. This is because of the excellent heat transfer characteristics of CO₂ and the relatively higher working pressures. The system requires less amount of refrigerant than a typical HFC to accomplish the same amount of refrigeration resulting in higher efficiency.
• Secondly, you'll see that the operating pressures are higher than what you would typically see in a supermarket. This is nothing to be afraid of as these pressures are going to be very similar to the pressures you will see in new air-conditioning systems that use R-410A. Most CO₂ systems will operate below 500 PSIG.
• Third, the pressure relief system is different in that there are two sets of reliefs. One relief system is a regulating system to keep the pressures below the maximum operating pressure by allowing small amounts of CO₂ to be vented in the case of a pressure spike that will result in a cooling effect of the CO₂ and lowering of the pressure. The second relief system is the standard relief system used today for rapid release of refrigerant in case of a sudden increase of pressure as from a fire.
• The fourth main difference will be the heat exchangers. Subcritical CO₂ systems use a refrigeration circuit from one refrigeration system (commonly referred to as a ‘Rack’ or ‘Top Cycle’) to condense or liquefy the CO₂ in order to make it useful for refrigerating. These heat exchangers will commonly be plate-to-plate type heat exchangers similar to those used on propylene glycol systems.

One key item to keep in mind with CO₂ is that it must be refrigerated at all times, or the pressures will exceed that of the relief valves and you will lose the charge. So, a backup emergency system is highly recommended, to keep the CO₂ vapor’s condensed in case of a power outage.

The Benefits of Using CO₂
CO₂ is an efficient natural refrigerant that is environmentally-friendly. CO₂ that's used as a refrigerant doesn't cause any harm to the environment, as it is already produced in some other process (e.g. hydrogen production) and it is effectively a deferred release. Therefore, you can vent it directly to the atmosphere with no requirements for reclamation. It is much lower in cost than the commonly used HFC’s. By using CO₂, you can significantly reduce the amount of HFC refrigerant used in a store. And the big benefit with CO₂ is the improved environmental impact of the system. Moderate to significant improvements in carbon footprint and energy performance improvements are achievable with CO₂. At Kysor/Warren, we are committed to developing solutions to the refrigeration challenges our industry faces today. That’s why we have developed Systems that utilize CO₂ for low temperature DX and MT secondary refrigeration applications. With the environmental awareness within our industry and its proven commitment to sustainable technologies, the demand for CO₂ refrigeration will no doubt increase. With the growing concern over the impact of refrigerant leaks into the environment, it is important that grocery retailers and equipment manufacturers partner to create ‘green’ solutions within the supermarket industry. By utilizing a natural refrigerant such as CO₂, the effective global warming potential can be lowered by approximately 40% to even 75%, creating a significantly less harmful impact on the environment – a win-win situation for both the refrigeration industry and more importantly, our eco-system.

Travis D. Lumpkin is vice president and systems business unit leader for Kysor/Warren. Kysor/Warren, a division of Manitowoc Foodservice, is a top manufacturer of refrigerated merchandise display cases and refrigeration systems for supermarkets and grocery retailers throughout North America. Built on a tradition of excellence, Kysor/Warren has been a part of the commercial refrigeration industry for more than 125 years and is a leader in innovative, sustainable refrigeration technologies. In October 2009, Kysor/Warren unveiled its first CO₂ cascading system with grocery retailer, Food Lion, at a store in Columbus, GA. More recently, the manufacturers’ environmentally-friendly CO₂ refrigeration system helped Fresh & Easy Neighborhood Market, Rosemead, CA, to open its first GreenChill certified store. In September 2010, Kysor/Warren was presented with the 2009-2010 GreenChill Distinguished Partner Award by the U.S. Environmental Protection Agency. The award recognized the company's commitment to protecting the environment through the manufacturing of eco-friendly products.

For additional information, visit Kysorwarren.com.